This invention relates generally to voltage controlled oscillators and is directed to a voltage controlled oscillator particularly adapted for implementation in integrated circuit form.
A voltage controlled oscillator (VCO) is an oscillator whose frequency is proportional to an externally applied voltage. VCO's are perhaps most commonly utilized in RF phase locked loop (PLL) tuning systems. When the phase locked loop is locked on an incoming periodic signal, the VCO frequency is exactly equal to that of the incoming signal. If the frequency of the incoming signal shifts slightly, the phase difference between the VCO signal and the incoming signal will begin to increase with time. This will result in a change in the control voltage applied to the VCO in such a way as to bring the VCO frequency back to the same value as the incoming signal. Thus, the phase locked loop maintains lock when the input frequency changes and the VCO input voltage is proportional to the frequency of the incoming signal. This behavior makes PLL's particularly useful for the demodulation of frequency modulated (FM) signals, where the frequency of the incoming signal varies in time and contains the desired information.
A common prior art voltage feedback tuned oscillator consists of a transformer, a capacitor and a pair of transistors providing current paths for each half cycle of AC output. This form of tuned oscillator is simple and reliable. However, the tendency today is to provide VCO's in integrated circuit (IC) form which offers the advantages of reduced power consumption, smaller size, higher operating frequencies, and increased reliability. The tendency is to further simplify and reduce the cost of VCO IC's by reducing the number of pins associated therewith. For example, this type of VCO IC is typically driven by a crystal oscillator requiring various input pins to the VCO. By reducing the number of pins associated with the IC, its cost and complexity can be similarly reduced making it available for a greater variety of applications.
The present invention is intended to provide such a VCO IC which requires a minimum number of crystal oscillator input pins, is capable of operating at video frequencies greater than 10 MHz, and offers the advantages of AC signal amplification from a positive feedback path and DC bias stability from a separate negative feedback path without use of external decoupling components.